1. Field of The Invention
This invention relates to n-type semiconducting diamond, and to a method of making the same.
2. Description of The Related Art
Semiconducting (doped) diamond has a number of characteristics which render it theoretically attractive for high-frequency, high-power semiconductor device applications. Such properties include a low dielectric constant, high electrical field breakdown voltage, elevated temperature stability, high electron and hole mobilities (electrons and positively charged carriers being nearly equally mobile), high thermal conductivity, and good radiation hardness.
A variety of techniques for forming diamond have been developed (see "Emerging Technology of Diamond Thin Films," Chemical and Engineering News, May 15, 1989, pages 24-39), including high pressure, high temperature synthesis (catalytic and non-catalytic); shockwave synthesis; and chemical vapor deposition (including direct-current plasma chemical vapor deposition, microwave plasma chemical vapor deposition, and heated filament-assisted chemical vapor deposition).
Semiconducting natural or synthetic diamonds are most commonly found or prepared as p-type materials, with boron atoms being the most common impurity species. See, for example, "The Properties of Diamond," edited by J. E. Field, Academic Press, London, 1979; and "Physical Properties of Diamond," Chrenko, R. M. and Strong, H. M., General Electric Report 75CRD089, October, 1975. It is widely known that the introduction of gas phase boron-containing species, such as diborane, during the diamond growth process will produce p-type diamond in which the majority carriers are holes. See "Characterization of Conductive Diamond Film," Fujimori, N., Imai, T., and Doi, A., Proc. ISIAT, 1985; and "Characterization of Conducting Diamond Film," Fujimori, N., Imai, T., and Doi, A., Vacuum, Vol. 36, 99, 1986.
Until the discovery of the present invention, however, it has not been possible to produce n-type semiconducting diamond via low pressure techniques using impurities other than nitrogen. Nitrogen, however, is not an acceptable donor species. Nitrogen forms a deep donor in diamond, such that the energy level of nitrogen in the diamond bandgap is too far (approximately 1.4 eV) below the conduction band minimum to be useful for the fabrication of practical semiconductor devices. Accordingly, the deep character of the nitrogen impurity level relative to the diamond conduction band minimum renders it virtually impossible to develop any usable carrier concentrations of sufficient magnitude at useful operating temperatures in semiconductor device application.
Other n-type impurity species such as arsenic and lithium that yield n-type diamond have been incorporated in previously formed diamond crystal lattices by ion implantation techniques. See "Semiconducting Diamonds," Vavilov, V. S., and Konorova, E. A., Sov. Phys. Usp., Vol. 19, 301, 1976; "Synthetic Diamonds In Electronics (review)", Bazhenov, V. K., Vikulian, I. M., and Gontar, A. G., Sov. Phys. Semicond., Vol. 19, 829, 1985; "Bipolar Transistor Action In Ion Implanted Diamond," Prins, J. F., Appl. Phys. Lett., Vol. 41, 950, 1982; "Electrical Properties of Ti and Cr Ion Implanted Diamonds Dependent on Target Temperature," Sato, S., Iwaki, M., and Sakairi, H., Nuc. Inst. Meth. Phys. Res., B19/20, 822, 1987; "Semiconducting Diamond Technology," Yoder, M. N., Naval. Res. Rev., Vol. 2, 27, 1987; and "Implantation of Antimony Ions into Diamond," V. S. Vavilov, M. A. Gukasyan, E. A. Konorova and Yu. V. Milyutin, Soviet Physics-Semiconductors, Vol. 6, p. 1998 (1973).
The incorporation of n-type impurity species into diamond crystal lattices by ion implantation, however, incurs the severe disadvantages of the implanted ions producing a heavily damaged surface layer which cannot be annealed away, and the implanted crystal needing to be post-implantion heat treated in order to electronically activate the implanted impurity.
Even with post-implantation heat treatment for electronic activation of the implanted impurity species, the highly damaged layer produced by ion implantation and the inhomogeneity and substantial concentration gradients of the ion implanted species across the implanted film thickness render the resulting n-type diamond wholly unsuitable for semiconductor device applications.
See "Distribution of the Conductivity With Depth in Diamond Doped by Bombardment With 10-45 Kev Li Ions, V. S. Vavilov, V. V. Galkin, V. V. Krasnopevtse and Yu. V. Milyutin, Soviet Physics-Semiconductor, 4, 1000, 1970; "Intrinsic Limitations of Doping Diamonds by Heavy-Ion Implantation," R. Kalish, M. Deicher, E. Recknagel and Th. Wichert, J. Appl. Phys., 50, 6870, 1979; "Spatial Distribution of Impurities and Defects Introduced in Diamond by High Energy Ion Implantation," V. S. Varichenko, A. M. Zaitsev and V. F. Stelmakh, Phys. Stat. Sol., 95, K123 (1986); and "Depth Profile of Antimony Implanted Into Diamond," G. Braunstein, J. Bernstein, V. Carsenty and R. Kalis, J. Appl. Phys., 50, 5731, (1979).
Accordingly, it is an object of the present invention to provide, for the first time, an n-type semiconducting diamond which is usefully employed in n-type semiconductor device applications.
It is another object of the invention to provide an n-type semiconducting diamond material which is devoid of the gross morphological defects characteristic of prior art ion implantation techniques for incorporating n-type impurity species in diamond lattices.
It is another object of the present invention to provide a method of in-situ doping of diamond with n-type impurity atoms during the formation of the diamond.
Other objects and advantages will be more fully apparent from the ensuing disclosure and appended claims.